Function coloured 

pub fn coloured(
    text: &str,
    spans: &[(Range<usize>, HighlightKind)],
    base: &TextRun,
    palette: &SyntaxPalette,
) -> Vec<TextRun>

One run per span, the text between them in the field’s own colour.

The runs a field paints TextField::set_spans as, public for the same reason next_boundary is: anything shaping the same text with the same palette wants the same answer, and this is where it is decided.

The spans are whatever the caller last handed over, which may describe text this frame no longer holds — see TextField::set_spans. A range that runs past the end, overlaps the one before it, or cuts a character in half is dropped rather than shifting the runs after it: shaping needs the lengths to add up to the text, and a colour that is wrong for one frame is cheaper than a panic.

base.len is ignored: every run is measured off text.